Closes #387

See websharks/quick-cache#387

- [x] Add new filter to allow site owner to disable cache locking
- [x] Add new filter to allow site owner to specify desired cache locking method (`flock` vs `sem`)
- [x] Change default cache locking priority to `flock()` → `sem_get()`
- [x] Update changelog with proper filter names (they changed after Jason's suggestions below)